Attaching Custom Data to the blend file, scene, objects
Only ID types, Bones and PoseBones support properties:


  Custom properties can be added to any subclass of an ID, Bone and PoseBone.


http://www.blender.org/documentation/blender_python_api_2_70_release/bpy.props.html

The only type with just a single instance is bpy.types.WindowManager, but its properties aren't serialized to .blend.